The gene expression for primate retrovirus, human chorionic gonadotropin, and placental growth factors was studied. A radioimmunoassay for the major internal structural protein (p26) from Macaca arctoides type C virus (MAC-1) was used to score antigen expression in rhesus placenta and other fetal organs obtained at various times of gestation. Antigen was detected in 16 of 16 placental specimens but not in 10 other different fetal organs from each of 8 selected animals. The levels of antigen in placenta ranged between 2 and 218 ng/mg protein with a corelation between lower antigen expression occurring with term gestation or parity greater than 10. Cocultivation of isolated trophoblasts from 10 rhesus placentas with 3 indicator cell lines (A549, FEC and CE2Th) led to rapid isolation in FEC of type C rhesus retrovirus in 4 of 10 cases. With all 4 isolates, p26 expression was detected in cell monolayers between 2-5 weeks, and MN++-dependent DNA polymerase activity was evident between 5-9 weeks. Previous isolates from Macaca (MAC-1 and MMC-1) were obtained in single long term experiments (over 7 months). Consequently, primary trophoblasts represent a useful differentiated cell source for isolation of infectious retrovirus from a "low producer" primate species.